Dual Lebanese-Saudi national Mr Hariri announced his resignation in a pre-recorded clip broadcast on Saudi Arabian television last week which said he feared for his safety.

It has prompted speculation that he was forced to stand down by Saudi authorities as a result of his unwillingness to confront the Iranian-linked Hezbollah militant group, which is fiercely opposed by the Saudis.

Lebanese officials have said his resignation was not accepted because it was declared in Saudi Arabia.

In Sunday's interview, Mr Hariri said his resignation was his decision, dismissing reports he was forced to quit a unity government with his political rivals Hezbollah.

Mr Hariri said he quit to save the country from imminent danger, but did not elaborate on what the details might be.

The British Foreign Secretary Boris Johnson said the UK offered its " full support for the sovereignty and independence of Lebanon" and he hoped Mr Hariri would return "without further delay".

He added: "We echo the concerns of the United States and our European partners that Lebanon should not be used as a tool for proxy conflicts, and that its independence and integrity should be respected by all parties - within the country and beyond."